The possibility of using liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transition or variable valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from 100.degree. to 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone, such as methylethyl ketone, or an aldehyde, such as acetaldehyde. Unfortunately, such aldehyde or ketone promoted variable valence metal catalysis was useful only for converting mono-, di- or trimethylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic and dimethyl benzoic acids. Two separate, later, and somewhat parallel lower temperature (80.degree.-100.degree. C.) modifications of the aldehyde or ketone promoted cobalt catalysis in liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid, but only at the expense of using rather high concentrations of cobalt with respect to p-xylene.
The disadvantages of using high concentrations of cobalt promoted with large quantities of aldehyde or ketone were overcome and, at the same time, a greater choice of variable valence metal oxidation catalysts were made available and a wider choice of alkyl-substituted benzene starting materials for benzene di-, tri- and higher carboxylic acids was provided by the discovery of the unique promotional effect on said variable valence metal by bromine ion, provided per se or formed in situ with or without acidic reaction medium provided by C.sub.1 -C.sub.8 monocarboxylic acids having no hydrogens on a tertiary carbon, such as benzoic acid and the saturated aliphatic monocarboxylic acids, preferably acetic acid. Such bromine-variable valent metal catalysis, first disclosed in U.S. Pat. No. 2,833,816.
The bromine-polyvalent metal catalysis in acetic acid solvent has been in commercial use in many countries for the manufacture of terephthalic acid from p-xylene for many years. However, in the absence of acetic acid solvent, best yield of a single phthalic acid (e.g., terephthalic acid) on a once through basis of the xylene, amounted to about 20 weight percent (12.8 mole), according to U.S. Pat. No. 2,833,816. According to U.S. Pat. No. 3,920,735 the Mn-Br and Co-Mn-Br catalyst system is improved by the addition of zirconium. However, not mentioned, but illustrated in Tables I, II and IV in U.S. Pat. No. 3,920,735, is the fact that, when part of the zirconium is added, combustion of the feedstock to carbon dioxide increases.